1. Field of the Invention
The present invention relates to the transfer, storage and delivery of process chemicals. More particularly, the present invention provides improved process and apparatus for the transfer, storage and delivery of ultra-high purity chemicals for use in a variety of industries, such as in the manufacture of semiconductor wafers and similar products, and electronic control thereof.
2. Description of the Prior Art
In many applications in industry today it is extremely important to maintain process chemicals free of virtually all contaminants. For instance, in the semiconductor industry the purity of chemicals, such as sulfuric acid, hydrogen peroxide, and ammonium hydroxide, used in semi-conductor wafer production must be pure on level of approximately 25 (or fewer) particles per milliliter with a particle size of less than a fraction of a micron. As a result of these purity standards, many conventional methods of chemical transfer and delivery, such as paddled pumps and similar devices, have proven completely unsatisfactory.
Of further concern in these industries is that many of the chemicals employed are toxic, chemically aggressive, and/or require special conduit material, and must be carefully handled. In order to assure adequate purity and worker safety, it is extremely important that such chemicals be transferred, stored, and dispensed in a closed system, with minimal contact with the environment or workers.
Prior to the invention of the parent application, generally one of two methods were employed to effectuate high-purity chemical transfer. The first method was a "pumped delivery." In this method a positive displacement pump, usually an air powered double diaphragm type, is employed to provide both lift at a suction inlet from the bulk source of the chemicals and simultaneous pressure at the output to the end-user. In this system, chemical is lifted from a chemical drum, driven through a pump, and pushed out to the point of use. Although this method continues to be widely employed, it is far from satisfactory.
The deficiencies of the pumped delivery system are manifold. This system is capable of producing only minimal lift from the chemical bulk source--usually on the order of only a few pounds per square inch. Moreover, this system is replete with contamination problems: the rapidly expanding and contracting of the pump diaphragm material (e.g. Teflon.RTM.) causes mechanical degradation, with the degradation by-products (many of which being too small to filter with state-of-the-art filtration equipment) entering the chemical process stream; further, the rapid action of the pump (usually greater than 60 cycle per minute) creates massive impulses in the system with a resulting pulsed flow which forces particles-through filters--thus rendering the filters far less effective. Finally, the mechanical shock and vibration inherent in this system creates constant maintenance problems, such as leaks.
The other system which was generally used addressed only some of these problems. In the "pump/pressure delivery," a positive displacement pump is again employed to provide lift from the bulk source of chemicals. However, the chemicals are delivered to an intermediate vessel from which inert gas pressure is used to motivate chemical to the use areas.
Although the pump/pressure system is better controlled and is more conducive to use of filters to assure chemical purity, it still has serious drawbacks in a sub-micron chemical environment. Again, lift provided by a double diaphragm pump is restricted. Further, such pumps are prone to degradation--with the by-products entering the chemical stream. Finally, the use of a single pressure vessel for delivery means that delivery is not continuous, but is rather constrained to "batch" sizes based on the size of the pressure vessel. If demand exceeds the volume of the pressure vessel, further delivery must be "queued" while the pump refills the pressure vessel. Alternatively, pressure from the pump that is equal to or greater than the pressure of the delivery vessel must be applied to the delivery vessel to supplement or refill it during demand; this further compounds the filtration and maintenance problems.
The invention disclosed in the parent application solves many of these problems. In that transfer method and apparatus, a combination of vacuum and pressure is used to smoothly transfer chemical from a bulk source, through one or more intermediate pressure/vacuum vessels ("PVV"), and to one or more end-use stations. First, a vacuum pump is used to establish a vacuum in one of the PVVs to draw chemicals into the PVV. Once a PVV is filled, the vessel is then pressurized to motivate chemical to an end-use station. By completely eliminating pumps from all chemical conduits in the system, the problems of degradation and contamination are avoided. Test results on prototypes of that system demonstrate that purity of the transferred chemicals is vastly improved over any other available chemical transfer apparatus.
The purity of this system is further improved by the elimination of pulsed flow through the transfer system. Unlike previous transfer systems where pumps or other transfer equipment causes a pulsed flow of chemicals (i.e. flow occurring at different pressures and velocities through the system due to the cyclical nature of diaphragm pumps or similar devices), the invention of the parent application employs a number of vessels arranged in parallel, allowing one vessel to fill while another is delivering chemical, so that chemical can be delivered without interruption or changes in flow rate. An even flow rate through the transfer apparatus provides far greater reliability in all aspects of the system, particularly in the use and maintenance of filters, again improving overall purity and dependability of the system.
The use of multiple, parallel vessels also provides tremendous flexibility in system operation. Among the advantages of such a system are the ability to by-pass a defective vessel without shutting down the system, and the ability to recirculate chemical through filters and/or back to a bulk source during periods of low or no demand while remaining ready to deliver chemical instantaneously upon request.
Despite the substantial advantages of the invention of the parent application, it is believed that significant improvements may be achieved in both the operation and control of such a system so as to utilize fully its potential for enhanced high purity chemical transfer.
Accordingly, it is a primary object of the present invention to provide an improved chemical transfer and delivery process and apparatus which effectively transfers high-purity process chemicals from any bulk source and delivers them reliably and without contamination to end-use stations.
It is an additional object of the present invention to provide such a process and apparatus which employs electronic controls to maximize the benefits of a vacuum/pressure delivery system so as to provide even flow at a consistent velocity with minimal mechanical shock in the system.
It is a further object of the present invention to provide such a process and apparatus which employs a solid state control system with a minimum of moving parts which may be subject to wear or degradation.
It is yet another object of the present invention to provide such a process and apparatus which employs electronic controls to maximize the benefits of a delivery system with multiple flow paths so to provide benefits such as: vastly increased delivery capacity; alternative flow paths to avoid system shut down in instances of failure of a component of the system; and automatic recirculation and/or filtration of chemicals.
It is another object of the present invention to provide such a process and apparatus which employs electronic controls to assure system reliability and to provide user feedback as to the status of the system and its components.
These and other objects of the present invention will become evident from review of the following specification.